The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Murderer cries while testifying in WA

A convicted murderer has wiped away tears while describing the night she witnessed a man strangle a Perth drug dealer with wire before helping him dump the body in water.

Allan Bradley McIntosh, 38, is on trial in the West Australian Supreme Court accused of murdering 46-year-old Diana Lesley Matthews, who was found in a reservoir at Rockingham Regional Memorial Park in April 2011.

Rebecca Anne Hall, who has already been sentenced for Ms Matthews' murder, testified on Wednesday that she drove McIntosh to a park where they met with the victim to buy drugs.

Afterwards, McIntosh convinced Ms Matthews to go with them so he could get money to buy more drugs from her because she would not give him amphetamines on credit.

Hall said Ms Matthews sat in the front passenger seat while McIntosh sat behind her and used a wire to strangle the woman while Hall drove the car.

"She was groaning," Hall testified.

"I'm pretty sure he was yelling at her ... something about it being her fault."